Bifoil blades have both an airfoil-shaped upper surface and an airfoil-shaped lower surface, they may be constructed in various shapes and sizes and may be used for many aerodynamic applications. Many factors are important in judging the utility of a bifoil blade for a given aerodynamic application. Probably the most important of such factors are the weight of the blade, the strength of the blade and the simplicity and economy of its construction.
One of the applications for which the present invention is particularly useful is that of a wind blade. Windmills and wind-powered equipment have been known since ancient times, but until relatively recently, little emphasis has been placed on the economic development of such systems. Now, largely because of the rising cost of energy derived from fossil fuel sources and the uncertainties and possible dangers involved in nuclear sources, thought is being given to the development of a wind-powered turbine generator which would be competitive with other energy sources. For such a system to be efficient, it must utilize blades which are long, so as to sweep the greatest area, and they must be light. At the same time such blades must have substantial internal strength to withstand high wind and other potentially damaging forces.
U.S. Pat. No. 4,081,220 to Andrews shows a wind blade utilizing a composite fiber wound support member. The fiber wound support member of the Andrews blade is composed of multidirectional fiber which is wound on a removable mandrel or a nonremovable foam mandrel. This fiber wound support member forms the finished shape of the blade in the area of the blade tip. A trailing edge mandrel is attached directly to the fiber wound support member and the combined unit is then helically wound with fiber to produce the finished blade shape.
In other applications, such as a helicopter blade, stronger blades are required although they are generally shorter, but blade weight still remains a critical factor. Fiber composite windings have often been used in forming such blades. See, for example, U.S. Pat. Nos. 3,476,484 and 3,950,115, which, although they are chiefly concerned with blade root attachment means, show blades which utilize fiber composite windings. Likewise, U.S. Pat. No. 4,096,012 shows the fabrication of a rotor blade spar from fiber glass tape. It has long been known that layers of directional fiber will increase the strength of a blade. See in this regard, U.S. Pat. No. 3,768,760, which shows a blade covering of successive layers of directional fiber.
It is an object of the present invention to provide a bifoil blade for a given aerodynamic application which has the least amount of weight without sacrificing internal strength.
It is also an object of the present invention to provide a wind blade which is long, light and strong.
It is a further object of the present invention to provide a wind blade whose structural features and low cost will allow it to be used in a wind-driven power generation system which will be more competitive with other energy systems.
The foregoing, as well as other objects, features, and advantages of the present invention are pointed out with particularity in the claims annexed to this specification. Further, they will become more apparent in light of the following detailed description of the preferred embodiment thereof and as illustrated in the accompanying drawings.
According to the present invention, a bifoil blade is provided comprising an upper and a lower airfoil-shaped skin; means for joining the upper and the lower airfoil-shaped skins; said joinder of the skins forming the surface of the bifoil blade, a leading edge and a trailing edge for said bifoil blade; and a central support structure enclosed within said joined skins comprising at least two channels each comprising two unidirectional fiber composite bars and a fiber composite channel web joining said bars, said bars extending from the root of the blade to the tip of the blade, one bar of each channel being affixed to the inside surface of the upper airfoil-shaped skin and the other bar of each channel being affixed to the inside surface of the lower airfoil-shaped skin.
The present invention provides for the construction of a blade which is adaptable to various applications. Unidirectional fiber is known to provide the maximum strength in the direction of winding, and the present invention's array of four channels with two contacting the inside of the upper blade surface and two contacting the inside of the lower blade surface distributes this strength to best advantage. The channels, however, are not intended to form an independent structural unit capable of carrying all of the blade loads, but are intended rather to cooperate with the blade surface skin members and other possible cloth or wound composites, including integrally wound composites. Thus the present invention provides a blade achieving maximum strength with the minimum materials and, therefore, with the lowest weight.